1. Field of the Invention
The invention is directed to a method and apparatus for the preparation of nanoparticles of cerium oxide. In particular, the invention is directed to a method and apparatus for the preparation of cerium oxide nanoparticles having a narrow size distribution.
2. Background Information
Cerium oxide in the form of fine particles is useful as a catalyst for polymerization, for reforming fuels, and for abating polluting gas in automobile exhaust systems. The catalyst acts as an oxygen pressure regulator in the reduction of NOX to molecular nitrogen, the oxidation of hydrocarbons and carbon monoxide to water and carbon dioxide, and the conversion of H2S to H2 and S.
Cerium oxide has been used as a catalyst-component for the recombination of hydrogen and oxygen to water for sealed car batteries, which extends battery life. The oxide is a good ionic conductor and has been used as an electrolyte material for a solid oxide fuel cells and gas sensors, as discussed, for example, by Steele, B. C. H., Solids State Ionics, Vol. 12 (1984), p. 391. The oxide has a high dielectric constant and a high refractive index (useful for optical coatings), and can be used as an insulating layer on semiconductor substrates. Cerium oxide is also of interest as a catalyst in vehicle emissions systems, as discussed in Yao, Y. F. and Kummer, J. T., Journal of Catalysis, Vol. 103 (1987), p. 307, and has also found use as a solid oxide fuel cell electrolyte material, as shown in Mogesen, M., Sammes, N. M. and Tompsett, G. A., Solid State Ionics, Vol. 172 (2000), p. 63; in gas sensors, as described in Lampe, U., Gerblinger, J. and Meixner, H., Sensors and Actuators B—Chemical, Vol. 7 (1992), p. 787; in optical coatings, as described in Haas, G., Ramsey, J. B. and Thun, R., Journal of the Optical Society of America, Vol. 48 (1957), p. 324; in high-Tc superconductor structures, as discussed in Walkenhorst, A., Schmitt, M., Adrian, H. and Petersen, K., Applied Physics Letters, Vol. 64 (1994), p. 1871; and silicon-on-insulator structures and high storage capacitor devices, as shown by Chikyow, T., Bedair, S. M., Tye, L. and El-Masry, N. A., Applied Physics Letters, Vol. 65 (1994), p. 1030. Because of the relative hardness of the material, cerium oxide nanoparticles are also useful as an abrasive for fine polishing of surfaces of certain materials, such as quartz and silicon.
Some applications may benefit from using monodispersed cerium oxide nanoparticles, due to either possibly new properties when such particles are nanodimensional or the greater control in uniform structures. A sub-micron scale cerium oxide powder has been prepared and used to decrease the sintering temperature from 1500° C. to 1200° C., as described by Chen, P. L. and Chen, I. W., Journal of the American Ceramic Society, Vol. 76 (1993), p. 1577; however, there has been no report of any method for preparing cerium oxide nanoparticles having dimensions smaller than about 14.5 nm, and no report of cerium oxide particles having monodispersity. The electrical conductivity of multi-dispersed nanoparticles of cerium oxide prepared by a vacuum technique has been investigated by Chiang, Y. M., Lavik, E. B., Kosacki, I., Tuller, H. L. and Ying, J. Y., Electroceramics, Vol. 1 (1997), p. 7. The vacuum sputtering technique used by Chiang et al. usually yields cerium oxide particles of a large size distribution, which makes it very difficult to test and sort out particle-size effects on the catalytic process for certain reactions. Tsunekawa, S., Sahara, R, Kawazoe, Y. and Ishikawa, K, Applied Surface Science, Vol. 152 (1999), p. 53; and Tsunekawa, S., Ishikawa, K, Li, Z. Q., Kawazoe, Y. and Kasuya, Y., Physics Review Letters, Vol. 85 (2000), p. 3440, both claim to have prepared mono-sized nanoparticles of cerium oxide and reported lattice expansions with decreasing particle-size in a few nanosized cerium oxide particles. Both papers suggest that a decrease in the size of cerium oxide nanoparticles is accompanied by a significant increase in the lattice parameter. Neither paper, however, discloses the method of preparation of cerium oxide nanoparticles or the apparatus used.
The foregoing discussion shows that there is a need in the art for an efficient method and apparatus for preparing significant quantities of cerium oxide nanoparticles with a relatively narrow size distribution.